The invention relates generally to the probing of signals in a circuit and, more particularly, to a system and method for creating probe masks to facilitate reliable signal probing.
FIG. 1 is a side view of a VLSI chip package 120 on an interposer 110 ready to be mounted on a PCB (Printed Circuit Board). The PCB couples to the interposer 110 at PCB surface 100. The interposer 110 acts as a conductive system between chip package 120 and the PCB.
To perform electrical analysis of signals to and from chips 120 mounted on a PCB, vias connected to these signals are extended through the PCB at the point where the chip attaches to the PCB. Vias are holes with plating to electrically connect features through the PCB. Conductive pads connected to these vias are then placed on the back side of the PCB. A user can measure the signal by contacting the pad using a test probe, such as that associated with an oscilloscope.
The pads on the PCB are typically small, numerous and closely spaced. For example, FIG. 2 illustrates a section of PCB 210 having approximately 1000 pads within one square inch. Each pad 220 is one oval shown in FIG. 2. Within each oval pad 220 is a via 200. Because a probe is relatively large in size compared to the pads, it is difficult to ensure the correct pad is being probed and that the probe is stabilized upon the pad.
FIG. 3 shows oscilloscope probe 300. Probe 300 has a body 330, an end 310 about the size of a ballpoint pen measuring approximately two and one-half inches in length, and a probe tip 320 measuring approximately one-half an inch in length. Probe tip 320 is used to contact and measure a signal at pad 220 (See FIG. 2). Some probes are quite large because of the electronics in the probe bodies, making it difficult to see the chip or the pad being measured.
Currently, a user must count over and up the number of pads to locate the pad to be measured and then place the probe upon that pad. There are a number of disadvantages associated with this standard method of locating the pad and measuring the signal at the pad. For example, locating the correct pad by counting up a number of pads and over a number of pads often requires the use of a magnifying glass to accurately count the pads. Moreover, once the first pad is located and a probe is placed upon the pad, it is difficult to locate and probe a subsequent pad(s) using another probe(s) at the same time because each additional probe further obscures the pads, making counting and probing increasingly difficult. Even if the user is able to keep track of the identified pad(s) for measurement, the probe tends to slide across the pad during measurement because of the pad""s smooth surface. The pad is smooth because of the insulating solder that is silk-screened over the pads and vias. The probe""s large size in comparison to the small area being measured makes handling the probe awkward and difficult to move towards the pad to be measured without losing track of the pad. Further, the repeatability of taking measurements diminishes because once the probe is removed from the pad, the pad must be relocated by counting and then remeasuring. Accordingly, currently it takes approximately 30 minutes to isolate and probe one pad.
Embodiments of the present invention provide a system and method for creating a probe mask. In one embodiment, such masks are used as aids in locating and accessing a pad on a PCB to measure a signal at the pad using a probe. The preferred embodiment mask being a perforated insulating sheet having holes corresponding to the spacing of conductive pads on the PCB, and having created thereon markings to aid in signal measurement device to aid locating and accessing a pad on a PCB to measure a signal at the pad using a probe. In the preferred embodiment a processing system is used to assist in the creation, and/or identification, of the holes. The device being a perforated insulating sheet having holes corresponding to the spacing of conductive pads on the PCB.